[Cfp-interest 2599] Re: draft CFP response for NB comments and N3071

[Jim Thomas]

Here’s an update to the draft, intended to address the issues raised by Joseph. Added or changed text is highlighted.

https://wiki.edg.com/pub/CFP/WebHome/CFP_review_of_NB_comments-20230107.pdf

- Jim Thomas

> On Jan 6, 2023, at 6:03 PM, Joseph Myers <joseph at codesourcery.com> wrote:
> 
> On Fri, 6 Jan 2023, Jim Thomas wrote:
> 
>>> GB-164: I'll disagree with the disagreement here (I assume this will end 
>>> up needing to be discussed in the WG14 meeting), since the semantic change 
>>> in C2x (which I don't think was ever explicitly discussed as an intended 
>>> semantic change in a paper) would make errno-setting implementations of 
>>> various functions much more complicated and less efficient - detecting 
>>> overflow after the fact (in a way that's valid for previous versions of C) 
>>> by checking for an infinite results from finite arguments is very 
>>> straightforward, determining based on the arguments whether pow, fdim, 
>>> hypot or fma (for example) would overflow / whether a result of the 
>>> largest finite value is an overflow is much more complicated and 
>>> inefficient (and saving and restoring exceptions so as to test, for the 
>>> purposes of errno setting, whether the overflow exception was raised, is 
>>> also inefficient; such explicit manipulations of floating-point state are 
>>> liable to empty the processor pipeline).  (This concern is specifically 
>>> about errno setting rather than exceptions; ensuring the overflow 
>>> exception is raised for overflow to a finite number isn't a problem in the 
>>> same way.  
>> 
>> Could you determine overflow errno setting from the overflow 
>> floating-point exception?
> 
> See above.  If a function has to save and restore floating-point state 
> explicitly, it's liable to be bad for pipelined execution (so efficiency).  
> (And the "save and restore" part is necessary, since the overflow flag 
> might already be raised on entry to the function.)
> 
>>> GB-279: The wording of some of the changes isn't quite right (constexpr 
>>> isn't a storage *duration*, so wording shouldn't refer to it as such in 
>>> F.8.4 or F.8.5).
>> 
>> Rewordings might be:
>> 
>> In F.8.4 #1 change “for an object that has static or thread storage 
>> duration” to “for an object declared with storage-class specifier 
>> constexpr, static, or thread_local”.
> 
> That's not correct.  An object with static storage duration might have no 
> storage-class specifier, or only the extern specifier, if at file scope.
> 
>> In F.8.5 #1 change “of objects that have static or thread storage 
>> duration” to “of objects declared with storage-class specifier 
>> constexpr, static, or thread_local”.
> 
> Likewise.
> 
> -- 
> Joseph S. Myers
> joseph at codesourcery.com

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